The invention relates to a differential gear drive.
In, for example, a motor vehicle supported by one or more axles carrying ground-engaging wheels, the wheels at each end of those axles rotate at the same speed when the vehicle is travelling in a straight path. However, when the vehicle is travelling in a curve, the wheel at the outside of the curve of a pair of wheels on an axle has to travel further than the other wheel in the same time. This means that on an axle where the wheels are connected together without permitting relative rotation therebetween, the traction between the wheels and the ground must be broken for either or both of the wheels on that axle. The force necessary to do this can usually only be provided by increasing the forces generated between the steered wheels and the ground. In view of the connection between the two wheels which is necessary for transmission of driving torque this increase in forces will interfere with the steering of the vehicle. To permit maneuvering without inhibiting steering action it is usual to fit a differential gear drive into the driving axle.
This is satisfactory when the wheels at each end of the driving axle and driven through the differential gear drive have sufficient grip on the ground surface. However the differential gear drive is a gear train which transmits equal torque to both output shafts provided equal reaction is generated in each of those shafts to provide the other with a reaction member. Thus if one of the wheels is on relatively slippery ground where no significant reaction is available at that wheel, the application of power to the differential gear drive causes the shaft driving the wheel on slippery ground to rotate ineffectively, denying drive to the other driven wheel which may still have reasonable grip on the ground on which it rests. Thus the vehicle is unable to move.
One way in which this problem has been overcome is to lock the differential gear drive when a driven wheel commences to slip or spin, or when the ground conditions are such that wheel slip or spin would otherwise occur, and thereby to prevent the differential gear drive acting at all and instead causing the driven wheels to be driven together at the same speed. This causes the driven wheels to be connected together thereby preventing relative rotation therebetween and produces the undesirable steering effects set out hereinbefore. As a result, where the steering wheel has been turned before the differential gear drive is locked, the locked driven wheels would act in conflict with the sterring effect, thereby causing unpredictable steering behavior. Such locking of the differential is thus most undesirable.
Another attempt to overcome the problem has been to use viscously-constrained differential gears in which the resistance of the differential gear drive to further increasing the relative rates of rotation of the output shafts increases substantially in proportion to an increase in those relative rates of rotation. Thus the undesirable effects on the vehicle steering of such a device are less than those of the simple differential locking devices discussed hereinbefore they are of the same degree and direction and towards the limit, the differential gear drive would again be undesirably locked. Other prior proposals have included the use of automatic devices to divert torque to the wheel which retains grip by the use of clutch or free-wheel arrangements which are actuated as the gripping wheel slows down, as it must do with the loss of drive when the non-gripping wheel slips. However the non-gripping wheel accelerates much faster than the gripping wheel decelerates and so the engine must accelerate with it, so that inertia effects lead to substantial load and sudden off-center drive effects which inhibit vehicle control and necessitate a differential gear drive of great bulk and cost to deal with these undesirable effects.
The maximum speed ratio between the output shafts of a differential gear drive fitted to a vehicle is a fixed value for that vehicle and depends only upon the dimensions of the track, wheel-base and maximum available steering angle of the vehicle. Other prior proposals have sought to constrain the differential gear drive to operate only within this regime but these proposals have involved means which are unacceptably fragile and in passenger car applications unacceptably noisy. As a result no automatic devices which achieve this desirable regime are in use in motor vehicles. The only devices which are in use are of the fundamentally undesirable locking type already discussed.
It is an object of the invention to provide a differential gear drive in which the relative speed ratio between the output shafts is limited to that necessary for the vehicle concerned by automatically sensing the overspeeding of a wheel having inadequate grip and preventing a relative speed ratio greater than the ratio specified. It is a further object of this invention to provide such a differential gear drive in which the drive to the other driven wheel or wheels is continued as normal through the differential gears as a result of means associated with the overspeeding shaft maintaining an adequate reaction on that shaft and thus a maximum speed ratio between the spinning and gripping wheels so as to eliminate undesired effects on the steering while continuing smoothly and continuously to transmit drive to the gripping wheel.
According to the invention, a differential gear drive between two output shafts each coupled through respective differential gears of the gear drive to a common input member includes control means whereby the maximum speed ratio between the two output shafts is limited to that ratio which is necessary for the differential gear drive to perform its intended function, whereby overspeeding of an output shaft which is under-loaded is prevented and torque is transmitted to a loaded output shaft at all times by means of respective differential gears, the differential action being unconstrained until said maximum speed ratio between the output shafts has been reached.
The control means may comprise a separate control clutch for each output shaft acting between that output shaft and a speed reference member co-axial with said output shaft.
The speed reference member is preferably common to both output shafts.
Said clutch means is not capable of transmitting drive direct to the output member so as to by-pass the differential gear and said clutch disengages immediately upon the speed of the overspeeding output member tending to fall below said limiting maximum speed. These means enable the transmission of drive to the gripping output member throughout. The limiting speed selected must be substantially equal to or proportional to .sqroot.R or 1/.sqroot.R, where R is the maximum differential ratio required of the differential gear drive in a particular vehicle.